Joint Tx/Rx Signal Processing for Distributed Antenna MU-MIMO Downlink

An introduction of multi-user multiple-input multiple-output (MU-MIMO) to distributed antenna-based small-cell networks is considered a promising approach toward 5G mobile networks to enhance the sum throughput. However, the inter-symbol interference (ISI) caused by the channel frequency-selectivity, the inter-antenna interference (IAI), and the inter-user interference (IUI) degrade the MU-MIMO downlink throughput. This paper proposes two joint transmit-and-receive (Tx/Rx) filtering schemes (called BD-SVD and MMSE-SVD) for MU-MIMO downlinks using single-carrier (SC) transmission and orthogonal frequency-division multiplexing (OFDM) transmission. In BD-SVD, IUI is removed by block diagonalization (BD) at the transmitter side, and then, the equivalent channel after BD is transformed into IAI-free eigenmodes using singular value decomposition (SVD). On the other hand, in MMSE-SVD, BD is not used and the channel is directly transformed into eigenmodes. IUI and IAI are suppressed by a minimum mean square error (MMSE)-based precoding at the transmitter assuming that each receiver does eigenmode reception. Furthermore, in the case of SC downlink, ISI is suppressed by applying MMSE-based Tx power allocation (PA) and Rx frequency-domain equalization (FDE) to each eigenmode. Numerical results show that BD-SVD and MMSE-SVD achieve a higher sum throughput than conventional MMSE precoding.